For tissue products such as facial and bath tissue and paper towels, strength and softness are important properties to many consumers. The strength properties of a product can be expressed in terms of wet strength and dry strength. The dry strength is important from the standpoint of manufacturing, since the product must have sufficient strength to pass through various stages in the manufacturing where the sheet is unsupported and under tension. In the case of paper towels, for example, the dry strength must also be sufficient to enable a towel sheet to be detached from a roll of perforated sheets without tearing and to perform tasks in the dry state without shredding. The wet strength is particularly important because towels are routinely used to wipe up spills. As such, it is necessary that the towel hold up in use after it has been wetted. The amount of wet tensile strength developed using conventional alkaline curing wet strength resins, such as polyamide-epichlorohydrin (PAE) resins (i.e. Kymene(copyright) resins from Hercules, Inc.) has been found in practice to be a function of the dry tensile strength of the sheet. Depending upon the furnish, the resin addition level and the water chemistry conditions, the wet tensile strength is generally limited to about 30-40 percent of the dry tensile strength of the sheet. Thus, in order to make tissue or paper products with a high level of wet tensile strength, one has to also develop a high level of dry tensile strength.
Unfortunately, tissues and towels with high dry tensile strengths also exhibit high stiffness and therefore poor hand feel properties since the properties of softness (as characterized by low stiffness) and strength are inversely related. As strength is increased (both wet and dry strength), softness is decreased. Conversely, as softness is increased, the strength is decreased. A high wet/dry strength ratio is desired to provide superior durability when wet, while at the same time exhibiting low stiffness and desirable handfeel properties when dry.
Hence there is a need for a means to increase the wet strength/dry strength ratio while maintaining or decreasing the stiffness of the sheet.
It has now been discovered that the ratio of the wet tensile strength to the dry tensile strength of a paper sheet, such as an uncreped tissue or towel sheet, can be substantially increased by properly treating the furnish, including adding appropriate amounts of a debonder, a wet strength agent and a dry strength agent. This discovery provides the flexibility to produce a tissue or towel product with increased wet strength while maintaining the current level of stiffness or, alternatively, maintaining the current level of wet strength while reducing the stiffness.
Hence, in one aspect, the invention resides in a method of treating a papermaking pulp useful for making a paper sheet, the method comprising: (a) adding a quaternary ammonium debonder to the pulp in an amount sufficient to significantly reduce the dry cross-machine direction (CD) tensile strength of the sheet; (b) thereafter adding a wet strength agent to the pulp in an amount sufficient to provide the sheet with a ratio of the wet CD tensile strength to the dry CD tensile strength (hereinafter the xe2x80x9cWet/Dry Ratioxe2x80x9d) of 0.50 or greater; and (c) thereafter adding a dry strength agent to the pulp in an amount sufficient to increase the dry CD tensile strength of the sheet.
In another aspect, the invention resides in a method of treating an aqueous dispersion of papermaking pulp useful for producing an uncreped throughdried paper sheet comprising: (a) adding to the aqueous dispersion of papermaking pulp from about 5 to about 30 pounds of a quaternary debonder per metric ton of dry fiber; (b) thereafter adding to the pulp from about 5 to about 30 pounds of a wet strength agent per metric ton of dry fiber; and (c) thereafter adding to the pulp from about 5 to about 20 pounds of a dry strength agent per metric ton.
In another aspect, the invention resides in an uncreped paper sheet, such as a tissue or towel sheet, comprising from about 5 to about 30 pounds of a quaternary amine debonder per metric ton of dry fiber, from about 5 to about 30 pounds of a polyamide-epichlorohydrin wet strength resin per metric ton of dry fiber and from about 5 to about 30 pounds of a dry strength agent per metric ton of dry fiber, said paper sheet having Wet/Dry Ratio of 0.50 or greater and a machine direction stiffness of about 30 kilograms or less per 3 inches of width.
In another aspect, the invention resides in an uncreped paper sheet, such as a tissue or towel sheet, comprising from about 5 to about 30 pounds of a quaternary ammonium debonder per metric ton of dry fiber, from about 5 to about 30 pounds of a polyamide-epichlorohydrin wet strength resin per metric ton of dry fiber and from about 5 to about 30 pounds of a dry strength agent per metric ton of dry fiber, wherein the ratio of the Wet/Dry Ratio to the machine direction stiffness is about 1.5 or greater.
The amount of the quaternary ammonium debonder can be about 5 pounds or greater per metric ton of dry fiber, more specifically from about 5 to about 30 pounds per metric ton of dry fiber, still more specifically from about 10 to about 25 pounds per metric ton of dry fiber. Suitable quaternary ammonium debonders include those chemistries containing one or more aliphatic hydrocarbon groups designed to disrupt hydrogen bonding in a paper, tissue or towel product made from wood fibers. Particularly suitable quaternary ammonium debonders include imidazoline quaternary ammonium debonders, such as oleyl-imidazoline quaternaries, dialkyl dimethyl quaternary debonders, ester quaternary debonders, diamidoamine quaternary debonders, and the like. A specific suitable imidazoline quaternary is 1-methyl-2-noroleyl-3-oleyl amidoethyl imidazolinium methylsulfate available from Goldschmidt Corp. under the designation C-6027.
The amount of the wet strength agent can be about 5 pounds or greater per metric ton of dry fiber, more specifically from about 5 to about 30 pounds per metric ton of dry fiber, still more specifically from about 10 to about 25 pounds per metric ton of dry fiber. Suitable wet strength agents include all chemistries capable of forming covalent bonds with cellulose fibers. Alkaline-curing polymeric amine-epichlorohydrin resins, such as polyamide epichlorohydrin resins, poly(diallylamine) epichlorohydrin resins and quaternary ammonium epoxide resins are particularly advantageous. A particularly suitable wet strength agent is a polyamide-epichlorohydrin resin sold by Hercules, Inc. under the trademark Kymene(copyright) 6500.
The amount of dry strength agent can be about 5 pounds or greater per metric ton of dry fiber, more specifically from about 5 to about 20 pounds per metric ton of dry fiber. Suitable dry strength agents include all chemistries capable of forming hydrogen bonds with cellulose. These strength resins may include modified starches and gums, modified cellulose polymers and synthetic polymers, including modified polyacrylamide polymers. A particularly suitable dry strength agent is carboxymethylcellulose (CMC), such as one available from Hercules Inc. as Aqualon(copyright) CMC 7MCT.
As used herein, dry CD tensile strengths represent the peak load per sample width when a sample is pulled to rupture in the cross-machine direction. The sample must be dry and have been conditioned at 73xc2x0 F., 50% relative humidity for at least 4 hours prior to testing. Samples are prepared by cutting a 3 inch widexc3x975 inch long strip in the cross-machine direction (CD) orientation. The instrument used for measuring tensile strengths is an MTS Systems Synergie 100. The data acquisition software was MTS TestWorks(copyright) 3.10 (MTS Systems Corp., Research Triangle Park, N.C.). The load cell is selected from either a 50 Newton or 100 Newton maximum, depending on the strength of the sample being tested, such that the majority of peak load values fall between 10-90% of the load cell""s full scale value. The gauge length between jaws is 4+/xe2x88x920.04 inches. The jaws are operated using pneumatic-action and are rubber coated. The minimum grip face width is 3 inches and the approximate height of a jaw is 0.5 inches. The crosshead speed is 10 +/xe2x88x920.4 inches/min. The sample is placed in the jaws of the instrument, centered both vertically and horizontally. The test is then started and ends when the specimen breaks. The peak load is recorded as the xe2x80x9cCD dry tensile strengthxe2x80x9d of the specimen. Ten (10) representative specimens are tested for each product and the arithmetic average of all ten individual specimen tests is the CD tensile strength for the product.
Wet tensile strength measurements are measured in the same manner, but after the center portion of the previously conditioned sample strip has been saturated with distilled water immediately prior to loading the specimen into the tensile test equipment. More specifically, prior to performing a wet CD tensile test, the sample must be aged to ensure the wet strength resin has cured. Two types of aging were practiced: natural and artificial. Natural aging was used for older samples that had already aged. Artificial aging was used for samples that were to be tested immediately after or within days of manufacture. For natural aging, the samples were held at 73xc2x0 F., 50% relative humidity for a period of 12 days prior to testing. Following this natural aging step, the strips are then wetted individually and tested. For artificially aged samples, the 3 inch-wide sample strips were heated for 6 minutes at 105+/xe2x88x922xc2x0 C. Following this artificial aging step, the strips are then wetted individually and tested. Sample wetting is performed by first laying a single test strip onto a piece of blotter paper (Fiber Mark, Reliance Basis 120). A pad is then used to wet the sample strip prior to testing. The pad is a green, Scotch-Brite brand (3M) general purpose commercial scrubbing pad. To prepare the pad for testing, a full-size pad is cut approximately 2.5 inches long by 4 inches wide. A piece of masking tape is wrapped around one of the 4 inch long edges. The taped side then becomes the xe2x80x9ctopxe2x80x9d edge of the wetting pad. To wet a tensile strip, the tester holds the top edge of the pad and dips the bottom edge in approximately 0.25 inches of distilled water located in a wetting pan. After the end of the pad has been saturated with water, the pad is then taken from the wetting pan and the excess water is removed from the pad by lightly tapping the wet edge three times across a wire mesh screen. The wet edge of the pad is then gently placed across the sample, parallel to the width of the sample, in the approximate center of the sample strip. The pad is held in place for approximately one second and then removed and placed back into the wetting pan. The wet sample is then immediately inserted into the tensile grips so the wetted area is approximately centered between the upper and lower grips. The test strip should be centered both horizontally and vertically between the grips. (It should be noted that if any of the wetted portion comes into contact with the grip faces, the specimen must be discarded and the jaws dried off before resuming testing.) The tensile test is then performed and the peak load recorded as the CD wet tensile strength of this specimen. As with the dry CD tensile test, the characterization of a product is determined by the average of ten representative sample measurements.
As used herein, xe2x80x9cmachine direction stiffnessxe2x80x9d is equal to the measured slope of the stress vs. strain curve obtained from the machine direction, dry tensile measurement. Upon completion of each tensile measurement, the MTS TestWorks(copyright) 3.10 data acquisition system calculates the xe2x80x9cslopexe2x80x9d using the gradient of the least-squares line fitted to the load-corrected strain points falling between a specimen-generated force of 70 to 157 grams (0.687 to 1.540 N), divided by the specimen width. The reported stiffness of a sample is the arithmetic average of ten representative sample measurements.
Suitable uncreped throughdrying processes useful for making tissue and towel sheets in accordance with this invention are well known in the tissue and towel papermaking art. Such processes are described in U.S. Pat. No. 5,607,551 issued Mar. 4, 1997 to Farrington et al., U.S. Pat. No. 5,672,248 issued Sep. 30, 1997 to Wendt et al. and U.S. Pat. No. 5,593,545 issued Jan. 14, 1997 to Rugowski et al., all of which are hereby incorporated by reference.
Suitable papermaking fibers useful for purposes of this invention include both bleached and unbleached hardwood fibers, bleached or unbleached softwood fibers, bleached or unbleached recycled fiber, synthetic fibers, non-woody fibers and blends of these fiber types. For towel applications, bleached softwood kraft fibers or a combination of bleached softwood kraft and bleached softwood chemithermomechanical pulp (BCTMP) fibers are particularly suitable.
The consistency of the aqueous papermaking pulp suspension when the debonder, wet strength agent and dry strength agent are added to the pulp can be any consistency suitable for the papermaking process. Specifically, the consistency can be about 5 percent or less, more specifically from about 1 percent to about 5 percent, still more specifically from about 2 percent to about 4 percent.
The basis weight of the uncreped sheets of this invention can be about 10 grams or greater per square meter, more specifically from about 25 to about 60 grams per square meter (gsm), still more specifically from about 30 to about 50 gsm.
The geometric mean dry tensile strength of the uncreped sheets of this invention can be from about 500 to about 7000 grams per 3 inches of sample width, more specifically from about 1000 to about 4000 grams per 3 inches of sample width, and still more specifically from about 1500 to about 3500 grams per 3 inches of sample width.
The dry CD tensile strength of the uncreped sheets of this invention can be from about 3500 grams or less per 3 inches sample width, more specifically about 3000 grams or less per 3 inches sample width, more specifically about 2500 grams or less per 3 inches sample width, more specifically about 2000 grams or less per 3 inches sample width, more specifically about 1500 grams or less per 3 inches sample width, more specifically about 1000 grams or less per 3 inches sample width, and more specifically about 500 grams or less per 3 inches sample width.
The wet CD tensile strength of the uncreped sheets of this invention can be from about 400 grams or greater per 3 inches of sample width, more specifically about 600 grams or greater per 3 inches of sample width, more specifically about 900 grams or greater per 3 inches of sample width, more specifically about 1200 grams or greater per 3 inches of sample width, more specifically about 1600 grams or greater per 3 inches of sample width, more specifically about 1800 grams or greater per 3 inches of sample width, more specifically from about 400 to about 2000 grams per 3 inches of sample width, and still more specifically from about 800 to about 1800 grams per 3 inches of sample width.
The Wet/Dry Ratio of the uncreped sheets of this invention can be 0.50 or greater, more specifically 0.60 or greater, more specifically from 0.50 to about 1.00, still more specifically from 0.55 to about 0.80, and still more specifically from 0.55 to about 0.75.
The machine direction (MD) stiffness of the uncreped sheets of this invention can be from about 30 kilograms or less per 3 inches of sample width, more specifically about 25 kilograms or less per 3 inches of sample width, more specifically about 20 kilograms or less per 3 inches of sample width, more specifically about 15 kilograms or less per 3 inches of sample width, and still more specifically from about 5 to about 30 kilograms per 3 inches of sample width.
The ratio of the Wet/Dry Ratio to the machine direction stiffness can be about 1.5 or greater, more specifically from about 1.5 to about 4, and still more specifically from about 2 to about 4.